The driver's side headlight of my Loyale has gone through three bulbs in less than a year; the last of which lasted only three weeks. The bulbs do not appear damaged after they burn out, and I am careful not to touch the bulbs when installing them. I have not had any issues with the headlight on the passenger side (and that one even has some moisture in it). Any thoughts on why this could be happening? I have not done any voltage testing yet.

Thank you!

There is high resistance in the circuit.Probably at the plug.

Contrary to what you might think,this can make the current thru the bulb higher than normal.

This happens because the resistance of the bulb itself  changes w/temperature.

Initially,the bulb is cold/low resistance but quickly heats to a higher resistance.

When there is unwanted resistance in the wiring,the bulb heats less(more slowly too)and has less than normal R.

The overall effect is less circuit resistance and too much current thru the bulb.

I would change the plug.

Bad connection at the headlight connector? Moisture inside the housing? Just a couple of thoughts.

Also check the voltage with the engine running above idle.   I've had the regulator fail full on.  This could shorten bulb life, among other things. 

Thank you all for the info!

2 hours ago, naru2 said:

There is high resistance in the circuit.Probably at the plug.

Contrary to what you might think,this can make the current thru the bulb higher than normal.

This happens because the resistance of the bulb itself  changes w/temperature.

Initially,the bulb is cold/low resistance but quickly heats to a higher resistance.

When there is unwanted resistance in the wiring,the bulb heats less(more slowly too)and has less than normal R.

The overall effect is less circuit resistance and too much current thru the bulb.

I would change the plug.

Just ordered a new plug, I'm going to try this first!

I swapped the older metal relays whit the newer plastic relays from JY imprezas.

On our '99 Forester the plastic plugs that connect to the headlamp bulbs had some heat damage from poor factory wiring.  This allowed play in the connection that would quickly damage bulbs.

My solution was to install a jumper harness between the factory lamp plug and the bulb with a ceramic plug for the bulbs in place of the plastic OEM plug.  Filled the jumper harness with dielectric grease, zip-tied the new jumper harness so it wouldn't flop around and haven't replaced a lamp in about a year+.  Pretty happy with this band-aid fix.

I would replace the connectors in the plug or get new pigtails. Usually this is the cause of short lived bulbs in the headlights.

Run your hand along the entire lens glas to see if you have a chip that is letting water get in.